Personally I wouldn't donate a cake, I would donate a dollar amount towards a cake. I think people look at the $ a lot more and realize what kind of value they are getting instead of just a wedding cake.

If you do it though make sure you are specific on how many servings, if it includes delivery or not... how much notice you will need, etc... typical items for any wedding cake, but you don't want a call a week or two ahead for this "free" cake and not have time or resources to do it.

I go with the 'keep it simple' thing and I'd donate a gift certificate of (for example) $300 toward a wedding cake. It's just cleaner and simpler. Suppose they put a limitation of 100 guests and the winner is planning a wedding for 200 ... they may think "Well, I can't use this". Or if they are planning a wedding for 200, they may not even bid on the item (which is bad for the auction).

I donate certificates for cakes, cupcakes, and cookies all the time. I restict the number of servings, give an approximate $value for that # of servings, give them 1 year to use it, request no less than 2 weeks notice, and if they want more servings, I make them donate more money to the charity.

I have a cake due at the end of July. The woman won a certificate for 50 servings. She needs 150. Because of the info on the certificate, she's now making an additional large donation to the original charity.